add doc

Author: andygrove

benchmarks/tpc/engines/comet.toml

@@ -32,4 +32,5 @@ driver_class_path = ["$COMET_JAR"]
 "spark.shuffle.manager" = "org.apache.spark.sql.comet.execution.shuffle.CometShuffleManager"
 "spark.comet.scan.impl" = "native_datafusion"
 "spark.comet.exec.replaceSortMergeJoin" = "true"
+"spark.comet.exec.graceHashJoin.enabled" = "true"
 "spark.comet.expression.Cast.allowIncompatible" = "true"